Metallic tantalum powder is conventionally produced by reducing K.sub.2 TaF.sub.7 with sodium. The physical properties of tantalum powders, such as for example grain size or specific surface area, are controlled by the addition of inert salts, such as KCl, NaCl, KF, NaF. As the inert salt content increases, the finer is the resultant tantalum powder, i.e. the resultant metal surface area is enlarged. However, the throughput of tantalum metal in the reduction process decreases in accordance with the increasing inert salt concentration.
Once the salts have been leached out, the tantalum powder is dried and subjected to high temperature treatment under a vacuum or in an inert gas atmosphere in order to purify it further. During this agglomeration stage, the specific surface area is significantly reduced and the oxygen content of the powder distinctly increased. This oxygen content is lowered again by heat treatment with metals having a reducing action, in particular magnesium. This reducing agglomeration results in a further slight reduction in surface area. In order to optimise the electrical properties of the capacitors produced from these tantalum powders, the tantalum powders are combined with dopants containing phosphorus and/or boron.
The electrical properties of tantalum powders, such as the specific capacitance or residual current are tested on a pressed, sintered and then anodically oxidised, i.e. formed, anode. The specific capacitance, stated in .mu.FV/g, is a measure of the capacity of the capacitor and is directly proportional to the surface area of the metal. Residual current, stated in nA/.mu.FV, is an indicator of how well a capacitor holds its charge.
Capacitor powders having specific capacitance values of 18000 to 70000 .mu.FV/g are produced economically using the conventional industrial sodium reduction of K.sub.2 TaF.sub.7 in fused salt. In order to obtain the tantalum powders having a small primary particle size which are required for high capacity capacitors, it is necessary to perform sodium reduction of K.sub.2 TaF.sub.7 at a relatively high dilution (diluent salts KCl, KF, NaCl), which results in smaller agglomerates (secondary particle size 1 to 5 .mu.m at primary particle sizes of approx. 0.3 .mu.m). The small dimensions of the agglomerates make it necessary to perform thermal agglomeration of the tantalum powders (presintering), wherein, on the one hand, unwanted impurities are removed but, on the other, the specific surface area is further reduced. The most highly capacitive tantalum capacitor powders hitherto known are described in DE 195 36 013 A1. Specific capacitance values of up to 91810 .mu.FV/g are achieved in sintered anodes produced therefrom if the otherwise conventional thermal agglomeration stage is omitted. These tantalum powders contain disruptive impurities, such as for example fluoride, in concentrations of &gt;100 ppm. A proportion of the elevated fluoride content is eliminated during sintering of the anodes. The fluorides released in this manner cause thermal corrosion in the sintering furnaces. A tantalum powder produced according to Example 6 of DE 195 36 013 A1 thus has a fluoride content of 460 ppm and an Mg content of 200 ppm. A further disadvantage is the elevated residual current values of the sintered anodes produced therefrom.
As is known, residual current values may be improved by doping with nitrogen or combinations of nitrogen with other elements such as carbon or sulfur in medium and low capacitive powders having specific capacitance values of &lt;30000 .mu.FV/g. This is described in U.S. Pat. No. 3,427,132, U.S. Pat. No. 3,825,802, U.S. Pat. No. 3,984,208, U.S. Pat. No. 4,154,609 and U.S. Pat. No. 4,544,403.
In these documents, nitrogen doping is used to reduce the oxygen content of the powders, to increase reliability or to improve residual current.
U.S. Pat. No. 187,598 furthermore describes a process which, after deoxidation, results in surface nitridation at temperatures of below 500.degree. C. with nitrogen contents of &lt;1000 ppm and an improvement in residual current of up to 30%. This method is, however, unsuitable for doping higher nitrogen contents, as the Ta powder undergoes an uncontrolled conversion into tantalum nitride at a temperature of above 500.degree. C.
U.S. Pat. No. 5,448,447 describes a process in which nitridation is performed by nitrogen gas or magnesium nitride, but which also permits doping of higher contents. This nitriding process has the disadvantage that an air-sensitive substance with varying nitrogen contents must be used, which means that a certain nitrogen level may reproducibly and accurately be established only with difficulty. JP-A 231 644 discloses nitridation with ammonia at temperatures of 1100.degree. C.
However, all these processes are restricted to powders having capacitance values of at most 30000 .mu.FV/g and applications for elevated working voltages of &gt;16 V (forming voltage &gt;70 V). Nitrided powders having capacitance values of &gt;30000 .mu.FV/g have not hitherto been known.
One reason for this is that the stated processes exhibit the disadvantage that, in fine Ta powders having a relatively high surface activity (BET&gt;1.5, capacitance values &gt;30000 .mu.FV/g), nitrogen contents of greater than 500 ppm cannot homogeneously be incorporated, due to the poorly controllable exothermic reaction of nitrogen or gases containing nitrogen such as ammonia. As described in U.S. Pat. No. 187,598, the reaction proceeds to completion in an uncontrolled manner. Moreover, all these processes exhibit the disadvantage that an additional processing stage is necessary for the nitridation.
Very finely divided powders are obtained by the gas phase reduction of TaCl.sub.5 with hydrogen. This gives rise to substantially discrete powders which no longer flow freely. Due to the difficulty of processing such powders under industrial conditions, they have found no acceptance in capacitor technology.